JavaScript is disabled for your browser. Some features of this site may not work without it.

The reliability based design of composite beams for the fire limit state

Van der Klashorst, Etienne (2007-03)

Thesis (MEng (Civil Engineering))--University of Stellenbosch, 2007.

Thesis

In the past use was made of prescriptive design rules to provide for the fire limit state. Modern
Design Codes provide the scope and the means to design for fire in a performance based manner.
The Eurocode provides guidance on the actions on structures exposed to fire as well as methods
to predict the structural behaviour of elements in fire.
Structural designers can now incorporate the use of parametric fire curves to describe compartment
fires. These fire models are not an extension of the old nominal standard temperature
time curves. Parametric curves are analytical models that are based on natural fire behaviour.
The temperature in the fire compartment can be predicted in a scientific manner taking account
of fire loads, ventilation conditions and compartment characteristics.
The combination of rational fire models and temperature dependant structural behaviour enables
designers to predict whether elements will fail during a fire. This is an improvement on
the empirical prescriptive fire resistance ratings, used to date.
Multi-storey steel framed structures, with composite floors, were identified as structures with
high inherent fire resistance and robust behaviour. The composite beams in the floor structure
were identified as critical elements when subjected to fire. The deterministic design and the
reliability level of these elements were studied.
Deterministic fire design procedures are presented that can be used to design unprotected composite
beams for the fire limit state. The reliability of the deterministic design procedures was
evaluated through a First Order Reliability Method.
Parametric fire curves are suitable for reliability analysis due to the fact that they can be
described by stochastic variables. The fire load was determined to be the dominant variable
influencing the reliability level of the composite beams. The ventilation conditions of the fire
compartment also has important implications for the temperature development of the composite
beams.
The reliability analyses results show that reasonably sized composite beams can be used as unprotected
elements in smaller fire compartments with moderate fire loads. It was found that a
structural element’s total probability of failure can be improved by the use of active fire fighting measures. The benefit of active fire fighting measures can be quantified by considering their
probability of failure.
By use of conservative assumptions and basic knowledge of fire engineering principles, rational
design methods can provide safe and economical solutions for fire design of composite beams.